Cast polyurethanes have a wide range of applications due to their unique combination of valuable physical and mechanical properties. In general, these elastomers can be prepared by the extension reaction of either polyether or polyester based isocyanate prepolymers with diol or diamine type chain extenders. Chain extenders are used to extend the hard segments in an elastomer. Both aliphatic and aromatic diol chain extenders are known in the art of cast polyurethanes. The most common aliphatic diol chain extender is 1,4-butanediol ("BD"). Common aromatic diol chain extenders include, for example, the bis-(.beta.-hydroxyethyl) ethers of resorcinol and hydroquinone. Drawbacks to the use of BD and the hydroquinone ether, however, exist. For example, while the hydroquinone ether yields suitable properties in the finished product, it often can't be used or isn't desirable to use because it requires high temperatures during processing. BD is processable at lower temperatures, but often fails to yield products with the desired properties.
Both polyether and polyester based elastomers have excellent tensile strength, abrasion resistance, and load bearing characteristics far superior to other elastomeric materials. To achieve an optimum performance for a particular application, various chain extenders are being used with toluene diisocyanate ("TDI") and 4,4' diphenylmethane diisocyanate ("MDI") terminated prepolymers based on polyether or polyester polyols. In the case of MDI-terminated prepolymers, 1,4-butanediol ("BD") is the most commonly used chain extender. Though the physical and mechanical properties of elastomers based on butane diol chain extenders are excellent, these elastomers have limited thermal stability.
Addition of both aromatic diols such as the bis-(.beta.hydroxyethyl) ethers of resorcinol and hydroquinone to polyurethanes helps to maintain mechanical properties of the polyurethanes at elevated temperatures. Although bis-(.beta.hydroxyethyl) ether of resorcinol and bis-(.beta.hydroxyethyl) ether of hydroquinone possess similar molecular structures, they have different processing characteristics in the cured elastomers. For example, bis-(.beta.hydroxyethyl) ether of hydroquinone has a substantially higher melting point than bis-(.beta.hydroxyethyl) ether of resorcinol, about 102.degree. C. versus about 89.degree. C. In order to use bis-(.beta.hydroxyethyl) ether of hydroquinone in cast urethane applications, the mixture must be processed at temperatures higher than 100.degree. C. and possibly as high as 120.degree.-130.degree. C. If lower temperatures are used, then "starring" occurs due to localized concentration of bis-(.beta.hydroxyethyl) ether of hydroquinone in the elastomer system. To overcome the processing problems associated with bis-(.beta.hydroxyethyl) ether of hydroquinone, bis-(.beta.hydroxyethyl) ether of resorcinol is used, as its lower melting point allows for a more forgiving chemistry and greater processing ease.
Other discussions of common chain extenders are found, for example, in Mendelsohn et al., Rubber Chemistry And Technology, "Characteristics Of A Series Of Energy Absorbing Polyurethane Elastomers" Vol. 58, pp. 997-1013, April 1985; that article discusses the need for polyurethanes having specialized "soft-hard" engineering applications, such as damping vibration, mitigating shock, and also providing rigid structured members with "soft-hard" characteristics. Extenders, such as 2-ethyl-1,3-hexanediol ("EHD"), BD, dipropylene glycol ("DPG"), bis-(.beta.hydroxyethyl) ether of resorcinol, and bis-(.beta.hydroxyethyl) ether of hydroquinone, were reported as being used in polyurethanes. Mendelsohn et al., U.S. Pat. Nos. 4,485,719 and 4,604,940, further disclose elastomeric materials requiring specialized properties of both strength and rigidity for aerospace missile launch pads and flexible missile shock isolator pads. These materials used bis-(.beta.hydroxyethyl) ether of hydroquinone as the sole chain extender for their polyurethane formulations.
International Application WO 98/56845 discloses thermoplastic polyether polyurethanes prepared by reacting a diisocyanate with a hydroxy terminated polyether having a molecular weight of at least 1,400 and a glycol chain extender. The application does not disclose the use of diisocyanate prepolymers or their combination with a diol blend chain extender.
U.S. Pat. No. Re 31,671 discloses a thermoplastic polyurethane prepared by reacting an isocyanate with a polyoxypropylene polyoxyethylene block copolymer and a chain extender; the chain extender is selected from aromatic and aliphatic diols and mixtures thereof.
U.S. Pat. No. 5,545,706 discloses a polyurethane elastomer comprising a prepolymer reacted with a glycol chain extender having an isocyanate index of about 70 to 130. The prepolymer is prepared by reacting a polyisocyanate having a functionality of 2 to 2.2, a polytetramethylene ether glycol having a molecular weight of between 600 and 6000 daltons, and 1 to 10 weight percent of a hydroxyl-functional polyoxyalkylene monol. The patent does not teach or suggest the reaction of the prepolymers used herein in conjunction with a diol blend chain extender.
U.S. Pat. No. Re 31,671 discloses a thermoplastic polyurethane prepared by reacting an isocyanate with a polyoxypropylene polyoxyethylene block copolymer and a chain extender; the chain extender is selected from aromatic and aliphatic diols and mixtures thereof.
U.S. Pat. No. 4,120,850 also teaches polyether urethane polymers. The polymers are the reaction product of a difunctional copolymer of tetrahydrofuran and ethylene or propylene oxide, an organic isocyanate, and an aliphatic diol having 2 to 10 carbon atoms. The use of the present diol blend is not taught or suggested.
Cast polyurethanes, also known as cast elastomers, are much more "high tech" than their thermoplastic counterparts and are prepared in a significantly different manner. Thermoplastic polyurethanes are typically prepared by mixing an isocyanate compound, a polyol, and a chain extender in the same reaction vessel. The result is a molecule containing alternating isocyanate and diol groups, wherein the diol can be either the relatively short chain extender or the relatively long polyol. Thus, thermoplastic urethane polymers have a random configuration. Higher performance properties than those provided by thermoplastic urethane technology are often required. An isocyanate prepolymer is a compound in which a relatively long chain polyol is capped on either end with an isocyanate compound. Prepolymer molecules are linked together by means of a chain extender. The use of isocyanate prepolymers, rather than isocyanate alone, allows the user to engineer the final polymer by better distribution of the hard and soft segments of the polymer. "Soft" segment refers to the polyol segment within the prepolymer, i.e., the portion of the molecule between the two isocyanates of each prepolymer group. "Hard" segments refers to the remaining portion of the molecule from the end isocyanate on one prepolymer group through the chain extender to the beginning isocyanate on the next prepolymer group. Typically the hard segment contains the aromatic groups. Performance of a polyurethane depends on the phase separation between the hard and soft segments of the elastomer. Having hard and soft segments as distinct from each other as possible therefore yields optimum properties in the final product.
The cast polyurethane industry is looking for chain extenders that improve the processing capabilities and enhance the physical and mechanical properties of the cured materials. Thus, there remains a need for improved chain extenders for these and other applications.